1. Field of the Invention
The present general inventive concept relates to an apparatus and method to estimate a sampling offset, and more particularly, to an apparatus and method to estimate a sampling offset included in a differential demodulated signal, using a pilot sub-carrier, an information sub-carrier, and a data sub-carrier extracted from the differential demodulated signal.
2. Description of the Related Art
Recently, a variety of broadcast/communication systems have used an Orthogonal Frequency Division Multiplexing (OFDM) communication method. The OFDM communication method transmits data using a plurality of orthogonal sub-carriers. Meanwhile, a Differential Quadrature Phase Shift Keying (DQPSK) Modulation method is a differential modulation method for shifting 2 bits of data and transmitting four waves having different phases. An Integrated Service Digital Broadcasting-Terrestrial (ISDB-T) method, an Integrated Service Digital Broadcasting-Terrestrial Sound Broadcasting (ISDB-TSB) method, etc. are used in OFDM/DQPSK systems that adopt the OFDM communication method and the DQPSK modulation method.
FIG. 1 is a view illustrating a frame structure which is used in an OFDM/DQPSK system.
Referring to FIG. 1, a frame 1 FRAME includes a plurality of symbols. In view of a time domain, each symbol period is divided into a guide interval GI and a valid symbol interval VI. In view of a frequency domain, each symbol includes a plurality of sub-carriers. The plurality of sub-carriers include data sub-carriers for transmitting data, information sub-carriers for transmitting control information such as a code rate, etc., and pilot sub-carriers which are used for channel estimation, offset estimation, etc. As illustrated in FIG. 1, in the OFDM/DQPSK system, a number of pilot sub-carriers is very small and most sub-carriers are data sub-carriers.
FIG. 2 is a block diagram illustrating a part of a receiver which is used in an OFDM/DQPSK system.
Referring to FIG. 2, the receiver includes an analog-digital converter (ADC) 202 for converting an analog signal Rx received through an antenna (not illustrated) into a digital signal, a sampling offset compensator 204 for compensating for a sampling offset which is generated during analog-digital conversion, a Fast Fourier Transform (FFT) block 206 for performing a FFT, a delayer 207 for delaying symbols, a differential demodulator 208 for receiving a current symbol SI and the previous symbol SI−1 and performing differential demodulation on the current symbol SI and the previous symbol SI−1, and a sampling offset estimator 210 for estimating a sampling offset SO on a basis of sub-carriers Psc extracted from the differential-demodulated signal.
However, conventionally, since the sampling offset estimator 210 estimates the sampling offset SO on the basis of only a small number of pilot sub-carriers Psc, it is difficult to accurately estimate the sampling offset SO. In particular, existence of a fading channel results in the accuracy of estimation being further lowered.
FIG. 3 is a view illustrating a response characteristic of a fading channel. In FIG. 3, a horizontal axis represents carrier indexes and a vertical axis represents response magnitudes according to carrier indexes.
Existence of an ideal channel results in a response magnitude of the ideal channel will be constant regardless of carrier indexes. However, in an actual channel, a fading characteristic as illustrated in FIG. 3 appears. As illustrated in FIG. 3, a response magnitude corresponding to a pilot sub-carrier Psc belonging to a fading region is very small due to influence of fading. The problem which occurs when the sampling offset SO is estimated on the basis of only a small number of pilot sub-carriers Psc becomes more significant if the pilot sub-carriers Psc belong to a fading region. That is, the sampling offset SO which is estimated on the basis of the pilot sub-carriers Psc belonging to the fading region will be further inaccurate.